The present invention relates to a transmission circuit using a strip line for transmitting microwaves or the like.
In recent years, with the development of equipment that exploit microwaves, there has been a rapid increase in number of transmission circuits equipped with a strip line. An example of this strip line is a microstrip line 90, as shown in FIG. 6, which is so formed that a dielectric substrate 91 is sandwiched between a strip conductor 92 and a grounding conductor 93. In the microstrip line 90, which is a modification of the parallel planar type waveguide, an electric field E and a magnetic field H are given to between the strip conductor 92 and the grounding conductor 93 so that electromagnetic waves are propagated along the microstrip line 90 in a direction vertical to the drawing sheet of FIG. 6. The impedance of the microstrip line 90 depends on width W of the strip conductor 92, thickness h of the dielectric substrate 91, permittivity .epsilon. of the dielectric substrate 91 and the like. The microstrip line 90 of this type is planar-structured, small-sized and lightweight, suitable for making of economical transmission lines and superior in compatibility with semiconductor component parts, so that the microstrip line 90 is widely and commonly used primarily as transmission circuits for microwaves.
FIG. 7 shows a sectional view of a transmission circuit using this strip line. The transmission circuit comprises an internal metal chassis 101, a first grounding conductor 102 provided on the top surface side of the internal metal chassis 101, a first dielectric substrate 103 provided on the top surface side of the first grounding conductor 102, a second grounding conductor 104 provided on the bottom surface side of the internal metal chassis 101 and a second dielectric substrate 105 provided on the top surface side of the second grounding conductor 104. Connecting pins 121, 122 are inserted into through holes 151, 152, respectively, which are provided in the internal metal chassis 101, the first grounding conductor 102, the first dielectric substrate 103, the second grounding conductor 104 and the second dielectric substrate 105, and spaced from each other with a specified interval. Then, a metal shielding plate 140 for shielding microwaves is erected in a generally mid point between the through holes 151, 152 on the first dielectric substrate 103. The internal metal chassis 101 strengthens the first and second grounding conductors 102, 104 while the strength of connection of the connecting pins 121, 122 is ensured. Also, cylindrical insulating portions 141, 142 made of synthetic resin are externally fitted to the connecting pins 121, 122, respectively, so that the insulating portions 141, 142 prevent the connecting pins 121, 122 from contacting the internal metal chassis 101 and the first, second grounding conductors 102, 104.
Further, one end of a strip conductor 111 provided on the top surface side of the first dielectric substrate 103 is connected to an upper end of the connecting pin 121 by a soldering portion 131, while one end of a strip conductor 112 provided on the bottom surface side of the second dielectric substrate 105 is connected to a lower end of the connecting pin 121 by a soldering portion 132. The other end of the strip conductor 112 is connected to a lower end of the connecting pin 122 by a soldering portion 134, while an upper end of the connecting pin 122 is connected to one end of the strip conductor 113 provided on the top surface side of the first dielectric substrate 103 by a soldering portion 133.
In this transmission circuit using a strip line, as shown above, a microstrip line formed by sandwiching the first dielectric substrate 103 between the strip conductor 111 and the first grounding conductor 102, a microstrip line formed by sandwiching the second dielectric substrate 105 between the strip conductor 112 and the second grounding conductor 104, and a microstrip line formed by sandwiching the first dielectric substrate 103 between the strip conductor 113 and the first grounding conductor 102 are connected one to another in a crank shape via the connecting pins 121 and 122. Thus, a three-dimensional transmission circuit is made up, by which a microwave signal is transmitted so as to avoid the metal shielding plate 140.
In the transmission circuit using a strip line shown in FIG. 7, microstrip lines are bent into a crank shape by using the connecting pins 121, 122 so as to avoid the metal shielding plate 140. Therefore, the transmission characteristics would be not stable but variable depending on the connecting pins 121, 122, the fitting position and material of the insulating portions 141, 142 externally fitted to the connecting pins 121, 122, the inner diameter of the through holes 151, 152 into which the connecting pins 121, 122 are inserted, the thickness of the internal metal chassis 101 and the like. This would be a large cause of impedance mismatching. Solder quantity of the soldering portions 131 to 134 by which the connecting pins 121, 122 are connected to the strip conductors 111 to 113 may also be a cause of impedance mismatching. Accordingly, in this transmission circuit using a strip line, impedance of the strip line would differ each time a transmission circuit is formed, so that fine adjustment of impedance is necessitated, resulting in a problem that the mass production effect deteriorates considerably. Further, since the above transmission circuit using a strip line is of the structure that different types of materials are stacked up with the internal metal chassis 101 sandwiched by dielectric substrates from top and bottom, mechanical stress would concentrate on the connecting pins 121, 122 because of the differences in thermal expansion coefficient among the materials, resulting in a fear of development to large problems such as pattern peeling.